The concentration measurement of a target substance contained in a sample, e.g., a biological fluid is an analytical method widely used in medical and biochemical fields. For example, in the medical field, the measurement of glucose concentration in blood or urine is applied as an effective screening test to medical diagnoses of diseases, e.g., hypoglycemia and diabetes mellitus.
In general, a method through the use of detection of a chemical reaction with respect to a target substance in a sample is widely used as a measure to determine the concentration of the target substance at present. A method in which the sample after the reaction of the whole target substance has been completed is observed and a chemical reaction with respect to the target substance is determined on the basis of the observation is mentioned as one of such chemical reaction detection methods. However, according to this method, the waiting time until the reaction of the whole target substance in the sample is completed increases and it may be inefficient.
As for another method without being accompanied by such an inconvenience, a method in which an initial reaction rate is measured and a chemical reaction with respect to the target substance is detected on the basis of this measurement is mentioned. In this method, a sample is observed after a predetermined term is elapsed from start of the reaction of the target substance, and a total amount of reaction of the target substance at that point in time is determined. This method is effective in the case where the substrate concentration can be estimated from the reaction rate with high accuracy as with an enzyme reaction, for example, and is useful because it is not necessary to wait until the reaction of the whole target substance is completed.
Also, as for a technique used for quantitatively detecting the reaction of the target substance, a colorimetric determination method described in, for example, PTL 1 is mentioned. For example, PTL 1 describes a method in which in a saliva sugar measurement, a reduced coenzyme or hydrogen peroxide produced by using glucose oxidase-peroxidase allows other oxidation-reduction substance to take on a color and the measurement is performed with a microplate reader or a spectrophotometer.
Meanwhile, PTL 2 describes a configuration in which hydrogen peroxide assay reagents and a sample containing an analyte are combined into a droplet and the assay is conducted in the droplet to yield a detectable signal (such as color change). Also, it is disclosed that the above-described droplet may be surrounded by a filler fluid, e.g., silicon oil, that is immiscible with the droplet.